The flexible lattice of hydroxyapatite (HAP) allows for doping with ions widely varying in size and charge and these ions can impart a range of properties that could be harnessed for high technology applications. However, incorporation of combinations of ions, each of which endows HAP with different properties, has rarely been explored, let alone in combination with polymers. Carbonated hydroxyapatite (CHAP) dually doped with magnesium and selenite ions (Mg-Se-CHAP) was synthesized at different contents of the Mg2+ dopant and integrated within electrospun epsilon-polycaprolactone (PCL) microfibers. The formula MgxCa represented the ceramic component of Mg-Se-CHAP/PCL fibers, whose properties were studied for different values of the stoichiometric parameter x in the 0.0 <= x <= 0.5 range. The structural investigation indicated that the lattice parameter a decreased with the addition of Mg2+ from x = 0 to x = 0.3, at which point it reached its minimal value of 9.414 angstrom, while the lattice parameter c increased with the addition of Mg2+ from x = 0 to x = 0.3, at which point it reached its maximal value of 7.050 angstrom. The morphological properties depended strongly on the Mg2+ content, as the fibrous scaffolds became more networked, rougher on the surface, and less porous with the addition of Mg2+. All of these surface properties affected the human fibroblastic HFB4 cell response to these materials. While the cell viability tests indicated a perfectly safe response toward the cells after 3 days of exposure, the cell adhesion, and proliferation improved upon the addition of Mg2+ to the Se-CHAP phase, and infiltration into surface pores varied, again as a function of the Mg2+ content. The mechanical properties were also strongly affected by the Mg2+ concentration, with tensile strength, fracture toughness and elastic modulus all recording their highest values for the x = 0.2 composition, typically being a dozen times higher than the values recorded for the Mg2+-free, x = 0.0 composition. These results show that a range of composite scaffold properties combining a polymer as the main phase (88 wt%) and HAP as the secondary phase (12 wt%), including the microstructural, mechanical and biological, can be tuned by controlling a relatively subtle and inconspicuous compositional parameter-the content of the Mg2+ dopant incorporated in the structure of HAP. Despite the low content of this ion in the dually doped CHAP/PCL microfibrous matrix, ranging between the Mg2+/Ca2+ molar ratios of 1:100 and 1:20, the high dependency of the material properties on the concentration of Mg2+ suggests that this approach may warrant further investigation for potential clinical applications.